Electrolytic recording media



United States Patent 3,310,479 ELECTROLYTIC RECORDING MEDIA AmnonGoldstein, Forest Hills, and Allan R. McGreevy,

Elmhurst, N.Y., assignors to Fairchild Camera & In-

strument Corporation, Syosset, N.Y., a corporation of New York NoDrawing. Filed Feb. 6, 1963, Ser. No. 256,543

9 Claims. (Cl. 2042) This invention rel-ates to electrolytic recordingmedia and to solutions for the preparation of the same. Moreparticularly, it relates to electrolytic recording media and toimpregnating solutions containing a tetraborate sensitizer to increasethe optical density of the recorded mark and to impart other valuableproperties.

Electrolytic recording is recording by means of the electrolytic actionof an electric current in a suitable electrosensitive medium. Therecording current is localized to product marks corresponding to graphiccopy scanned at a remote point. In electrolytic facsimile recording itis common to dipose a recording sheet or electrolytic recording mediumbetween an anode and a cathode at least one of which is in motion. Theelectrolytic recording media is also drawn past the electrodes at aregular rate to produce linear recording. Various recording devicesincorporating the use of an electrolytic recording media are known andhave been described. A suitable device for use in accordance with thisinvention is described in US. Patent 2,575,959,

Various electrolytic recording media have been described. They comprisea porous sheet or support such as paper impregnated withelectrolytically conductive aqueous solution containing a markingcompound and an electrolyte. The preferred marking compounds arepolyhydroxy compounds such as oatechol. Other classes of compounds suchas aminophenols, organic acids and aldehydes having a polyhydroxylatedphenyl group in the molecule, reduced quinones and derivatives of thesecompounds such as the esters and. amides of the acids or the oximes ofthe aldehydes are useful. Aryl, alkyl or analkyl substituted analoguesof the aminophenols or of the acids, aldehydes or their derivatives mayalso be employed. Hydroquinone, salicylaldoxime, salicyclic acid,s-alicyclic aldehyde, o-aminophenol, ,hydroxylated aminotoluidines arementioned as examples of marking compounds othe than polyhydroxy markingcompounds.

While it is preferred to utilize polyhydroxy compounds in the practiceof this invention, it is specifically intended to include other markingcompounds .such as those described above within the scope of theinvention. With this in mind, the invention will be specificallydescribed and illustrated with reference to polyhydroxy compounds forease of understanding and simplicity of description.

When the recording media is sandwiched between a cathode and an anode,the latter being formed from iron, stainless steel, vanadium orequivalent metal and a current is passed, the anode dissolves into theconductive solution and reacts with the polyhydroxy compound to form amark whose tonal quality varies from pale gray to dark black dependingon the current intensity.

Several electrolytes which are useful in electrolytic recording mediaare known. Their function is to impart electric conductivity to theimpregnating solution. Their concentration is a particular solution is amatter of choice depending upon the degree of conductivity desired. Theconcentration may vary from as low as 1% by weight or even lower to ashigh as the solubility limit of the electrolyte in the solution. Thepreferred concentration is from about 5% to about 15% by weight. Typicalelec troly-tes used in the preparation of electrolyticrecording mediainclude alkali and alkaline earth metal halides and nitrates. The choiceof electrolyte is not critical to this invention but sodium or potassiumchloride, bromide or nitrate are preferred.

A wide variety of polyhydroxy compounds are used in the preparation ofelectrolytic recording media. These are phenolic compounds capable ofreacting with dissolved anode material to produce a mark. Onepolyhydroxy tion for special purposes.

compound which is widely used is catechol. Others which are usefulinclude pyrogallol, gallic acid, protocatechuic acid and tannic acid.Catechol although widely used is somewhat objectionable because of itshigh vapor pressure and of late, there has been considerable interest inreplacing it in commercial preparations with less volatile markingcompounds. Acids and their derivatives such as esters, amides orequivalents are of special interest because of their low volatility. Anyof the known polyhydroxy compounds utilized in the preparation ofelectrolytic recording media can be employed in the practice of thisinvention.

The preferred polyhydroxy compounds used in praparing electrolyticrecording media of this invention have at least two adjacent hydroxylgroups and it is believed that formation of the mark involves chelationof the dissolved anode material between these groups. It is notessential however, that the functional groups in the marking compound beadjacent.

The marking compound to be useful must be at least somewhat soluble inwater, say for example, at least 1% by weight at 20 C. Its concentrationis not critical, but may vary from about 1% by weight or even lower upto the maximum of solubility for the particular compound employed.Generally, the concentration varies between about 1.5% and 5% by weight.

Various additives such as stabilizers, acidifiers and antifringingagents may be added to the impregnating solu- Many examples .areknown.Oxidants such as potassium chlorate have also been employed to enhancethe intensity of oxidation marks.

Electrolytic recording media once prepared are generally not usedimmediately but are stored while still wet with the impregnatingsolution in sealed containers to retard evaporation. Unfortunately,during storage there is a tendency for the compositions to degrade. Ithas been the practice of the art to include relatively small amounts ofstabilizers to retard this degradation. Stabilizers generally used arethiocarbonyl compounds suhc as thiourea, alkyl derivatives of thioureaand dithioburet. Of these thiourea is preferred,

In addition to stabilizing the compositions, the stabilizers alsostabilize the recorded mark. Thus, when stabilizers are used there isless tendency for the mark to fade or discolor on prolonged standing.

It is aspecial advantage of the impregnating solutions and recordingmedia of this invention that smaller amounts of stabilizer can beutilized than has heretofore been necessary. In fact because of theremarkable stability of the media, it is often possible to omit thestabilizer completely especially in high volume operations where themedia will be used within a relatively short time, i.e. one to twomonths after preparation. If the media is to be stored for a longerperiod or if it is essential that the mark remain clear and readable fora long time, it is best, however, to use a. stabilizer. Generally, aslittle as 0.1% by weight will be used. Useful results are obtained atconcentrations of from about 0.05% to about 1.0% by weight.

at the maximum of solubility of the sensitizers.

objectionable for many purposes. The preferred range is from 2 to 5.Compositions prepared in accordance with this invention manifest a pHwithin the operable range. However, certain of them may be occasionallyimproved by adjusting the pH to a particular value. This may be effectedby adding the base or the acid of the salt employed as the electrolyte.Thus, if sodium chloride is used as the electrolyte, the pH may beadjusted when desired with sodium hydroxide or hydrochloric acid.

The preferred acidifier for use in this invention is oxalic acid. Othermineral acids or organic acids such as hydrochloric, nitric, sulfuric,citric, itaconic, formic and maleic acid for example are also useful.The use of an acidifier is not essential since many impregnatingsolutions with a pH in the above disclosed range can be prepared withoutadditional acids.

Antifringing agents are used as an aid for maintaining sharp detail inthe mark. Typical antifringing agents include alkali metal phosphatesand pyrophosph'ates such as monosodium and disodium phosphate andpyrophosphate and their potassium analogues. The use of antifringingagents is not essential for the recording media described herein butwhen utilized, they are generally present at a concentration of fromabout 0.01% to about 0.3% by weight.

In accordance with this invention novel electrolytic recording media andimpregnating solutions for their preparation are provided. The solutionis an electrolytically conducting solution containing an electrolyte anda marking compound as described above together with a tetraboratesensitizer. Any tetraborate salt which is suificiently soluble in theimpregnating solution to provide concentrations of up to by weight on ananhydrous basis at room temperature is suitable as a sensitizer. Sodiumtetraborate is preferred. Many tetraborates exist as hydrates.Pentahydrates and decahydrates of sodium tetraborate are well known.Potassium tetraborate exists as both pentaand octahydrate. For thisinvention, either hydrated or anhydrous forms of tetraborate can be usedand when reference is made to a tetraborate it is specifically intendedto include both anhydrous and hydrated forms. The concentration ofsensitizer is not critical. Useful results are obtained atconcentrations as low as 1% -by Weight or even lower and are similarlyobtained Preferred concentrations are from about 2% to about 7% byWeight on an anhydrous basis. Acidifiers, stabilizers and antifringingagents can be employed as desired.

Electrolytic recording media prepared as described herein have a numberof advantages over media prepared by prior known methods. Of these, themost important for many applications is the fact that the recorded imageis extremely stable and shows little or no fading or yellowing evenafter prolonged standing. In side by side tests in which the resultsobtained with media prepared by prior art methods were compared with theresults obtained using similar media prepared by the methods of thisinvention, it was found that after a month of storage, the image on theprior art media had faded considerably especially in the lighter areasand these had also developed a yellow tinge. In contrast, the image onthe media of this invention was still sharp and clear. These resultswere achieved despite the fact that the rior art impregnating solutioncontained 0.25 gram of thiourea stabilizer per 100 cubic centimeters ofsolution, whereas the solution used to prepare the media of thisinvention contained less than one-half of this quantity.

There is practically no streaking of the image with the media of thisinvention. Moreover, the avalanche effect is also negligible.Avalanching is a well known and recognized phenonemon. It is overmarkingin certain areas of the medium and causes non-uniform shading of themark. Much better tonal quality of the mark than is possible with priorknown recording media can be achieved 4, over a wide range of currentdensities and recording speeds.

The exact mechanism by which tetraborate sensitization takes place isnot known. Although tetraborates are oxidizing agents, sensitizationtakes place independently of this property. There are two types of marksrecognized in the recording art. These are oxidation marks andprecipitation marks. With oxidation marks optical density is improved bythe addition of an oxidizing agent such as potassium chlorate to theimpregnating solution and increases with increasing concentration. Thepresence of oxidizing agents has no effect on other types of marks.These latter marks are known as precipitation marks. Tetraboratesensitizers improve the optical density of both oxidation andprecipitation marks.

Impregnating solutions for use in this invention are prepared bydissolving the various ingredients in water. Several sample solutionsare shown hereinafter.

Electrolytic recording media may be prepared by passing a orous supportthrough a bath containing the impregnating solution. Approximately 2.5to 3 cubic centimeters of the solution is generally required per squarefoot of support. The solution is distributed evenly throughout thesupport. Any excess may be removed by compression for example betweenpressure rolls. When ready. for use, the electrolytic recording mediawill contain about 40% impregnating solution by weight. As aforesaid,the media are generally stored in sealed containers until needed.

The following sample solutions are given by way of example and are notintended as limitations of this invention many apparent variations ofwhich are possible without departing from the spirit and scope thereof.In each of the solutions, the indicated quantities of the variousingredients are dissolved in cc. of water. Useful electrolytic mediaapplicable to low, medium and high speed recording may be prepared fromany of them by adjusting the pH to form 1 to 6 as described above andimpregnating a suitable porous support such as paper.

Grams NaNO 10.0 Catechol 4.5 Na2B407.10H2O 4-0 Oxalic acid 0.125Thiourea 0.1

. B NaCl 16.0 Gallic acid 2.0

N212B4O7-10H2O Thiourea 0.5

. C Tannic acid 1.5 NaCl 8.0 Na B O .10H O 2.5 Thiourea 0.1

NaCl 7.0 Pyrogallol 2.0 Na2HPO4 Na2B O7- Thiourea 0.1

E Methyl gallate 1.0 NaCl 8.0 Na B O .10H O 2.5 Thiourea 0.1

NaCl 8.0

' Gallamide 1.0 Na HPO 0.1 Na B4O7.10H O Thiourea 0.1

G KNO 12.0 Catechol 4.5 Na2B4O7 2.2 Citric acid 0.125 Thiourea 0.1

H NaBr 3.0 Gallic acid 2.0 K B O .8H O 3.0 N-ethyl thiourea 0.7

1 NaCl 16.0 Gallic acid 2.0 Na2B407 7.0 Thiourea 0.5

J 7 NaCl 7.0 Pyrogallol 2.0 N34P207 Na2B4O pH20 2.0 Thiourea 0.2

K KNO 12.0 Pyrog allol 4.5 Na2B407 Thiourea 0.15

L NaNO 10.0 Catech-ol 4.5 Na B O .5H O 4.0 Itaconic acid 0.12

M NaCl a 16.0 Gallic acid 2.0 K B O .5H O 7.0

N NaNO 10.0

C-atechol 5.0 Na2B4O7.10H O H 30 (con.) 0.125 Thiourea 0.05

O NaCl 7.0 Gallic acid 2.0 NH4P207 3-0 N212B407-10H20 2-0 Thiourea 1.0

P NaCl 8.0 Gallamide 1.0 Na HPO 0.05 Na2B407. Thiourea 0.1

Q N-aBr 3.0 Gallic acid 2.0 K2B407.8H20 10.0 N-ethyl thiourea 0.7

R NaCl 8.0 Gallamide 1.0 NaH2PO4 Na2B4O7.10H O V 2.5 Thiourea 0.1

What is claimed is:

1. For use in electrolytic facsimile recording, a wet web comprising aporous sheet impregnated with an electrolytically conducting aqueoussolution containing at least 1% by weight of a phenolic marking compoundtogether with from about 1% to about 10% by weight based on theanhydrous salt of a tetraborate sensitizer.

2. A web as in claim 1 in which the marking compound is a polyhydroxyphenolic marking compound.

3. A web as in claim 2 in which the polyhydroxy marking compound isselected from the group consisting of catechol, pyrogallol, gallic acid,gallamide, methyl gallate, protocatechuic acid, and tannic acid.

4. A web as in claim 1 wherein the electrolytically conducting solutioncontains from about 0.05 to 1% by weight of a thiocarbonyl stabilizer.

5. A web as in claim 4 wherein the stabilizer is thiourea. 1

6. A wet web as in claim 1 in which the pH of the electrolyticallyconducting solution is maintained between 1 and 6 by the presence of anacidifier.

7. A wet web as in claim 6 in which the 'aeidifier is selected from thegroup consisting of hydrochloric, nitric, sulfuric, citric, itaconic,formic, maleic and oxalic acids.

8. A wet web as in claim 1 in which the electrolytically conductingsolution contains from 0.01 to 0.3% by Weight of an antifringing agentselected from the group consisting of alkali metal, phosphates andpyrophosphates.

9. A wet web as in claim 1 wherein the tetraborate is sodiumtetraborate.

References Cited by the Examiner UNITED STATES, PATENTS 2,367,113 1/1945 Gibney 204-2 3,152,903 10/1964 Shepard et al 96-64 OTHER REFERENCESThe Condensed Chemical Dictionary, Sixth Edition, Reinhold Company, p.1036 relied on.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

W. VAN SISE, Assistant Examiner.

1. FOR USE IN ELECTROLYTIC FACSIMILE RECORDING, A WET WEB COMPRISING APOROUS SHEET IMPREGNATED WITH AN ELECTROLYTICALLY CONDUCTING AQUEOUSSOLUTION CONTAINING AT LEAST 1% BY WEIGHT OF A PHENOLIC MARKING COMPOUNDTOGETHER WITH FROM ABOUT 1% TO ABOUT 10% BY WEIGHT BASED ON THEANHYDROUS SALT OF A TETRABORATE SENSITIZER.